In U.S. Pat. No. 3,500,712, there is disclosed a screw of this type having a head of hexagonal peripheral geometry as a torque-receiving means and a shank having a peripheral thread. The shank end facing the head has an unthreaded region onto which is fitted a washer assembly. The washer assembly includes a sealing washer made of an elastic material and a relatively rigid washer made, for example, of metal. The washer assembly is relatively close to, or directly contiguous to the underside of the screw head.